The invention relates to a circuit arrangement for rectifying a signal, in which two transconductors are controllable with opposite phase by means of the signal to be rectified.
In electronic devices, for example, in radio receivers, it is often necessary to gain information about the amplitude of a signal changing its sign (alternating voltage), either for driving controllers which react to the amplitude, or for regaining (demodulation) the information content from a received signal. This is usually effected by means of active rectifier circuits.
It is known from, for example, U.S. Pat. No. 5,721,507 to arrange two transistors in parallel in one branch of a differential amplifier, which transistors are impressed, in phase opposition, with the signal to be rectified. Dependent on the sign of the signal to be rectified, one or the other of the parallel arranged transistors is turned on. Due to the curvature of the base-emitter characteristic curve, disturbing non-linearities are produced for small amplitudes of the applied signal. These non-linearities are avoided in a full-wave rectifier circuit as described in DE 30 12 823, in which the phase-opposed signals to be demodulated are first amplified in a respective transconductor and then applied from the outputs of the transconductors to a resistor via a respective diode. One of the diodes is then conducting in dependence upon the sign of the signal to be rectified. However, it cannot be avoided that one of the output transistors of that transconductor which is connected to the blocked diode reaches its saturation state. This state is particularly detrimental in an integrated circuit because current flows into and out of the substrate and the relevant transistor gets out of its saturation state only with a delay. The behavior of the known full-wave rectifier circuit at higher frequencies is thereby non-linear.
It is an object of the present invention to provide a circuit arrangement for rectifying a signal, which operates accurately through a large frequency range and supplies a linear output signal.
According to the invention, this object is solved in that the outputs of the transconductors are connected to a first circuit point via a first diode and to a second circuit point via a second diode having an opposite polarity with respect to the first diode, in that the circuit points are connected to a predetermined potential, and in that a rectified signal can be derived from at least one of the circuit points.
The circuit arrangement according to the invention is basically suitable for differential signals, i.e. signals which are pairwise phase-opposed, as well as for signals to ground. The phase opposition in the control of the transconductors can be achieved in that two phase-opposed signals are applied to equal inputs of the transconductors, or that equally phased signals are applied to opposite inputs of the two transconductors. The predetermined potential can be chosen by those skilled in the art in such a way that, starting therefrom, a control in both directions is possible, i.e. the predetermined potential is approximately in the middle of the control range of the transconductors. Moreover, the relevant predetermined potential should be chosen for the first and the second circuit point in such a way that there is a smooth transition between the diodes.
In a first advantageous embodiment of the circuit arrangement according to the invention, the rectified signal can be advantageously gained as voltage signal in that at least one of the circuit points is connected to the predetermined potential via a current input of a current/voltage converter, whose output constitutes the output of the circuit arrangement. The current/voltage converter is advantageously constituted by a current mirror circuit.
In a second embodiment, a voltage signal is gained in that at least one of the circuit points is connected to the predetermined potential via a resistor and constitutes an output of the circuit arrangement.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.